Function Get-LastxOfMonth {
    [CmdletBinding()]
    param(
        [parameter(Mandatory)]
        [String]
        $Day,

        [parameter(ParameterSetName = 'ByDate',
            Mandatory,
            ValueFromPipeline)]
        [System.DateTime]
        $Date,

        [parameter(ParameterSetName = 'ByString',
            Mandatory,
            ValueFromPipelineByPropertyName)]
        [ValidateRange(1, 12)]
        [int]
        $Month,

        [parameter(ParameterSetName = 'ByString',
            Mandatory,
            ValueFromPipelineByPropertyName)]
        [ValidatePattern('^\d{4}$')]
        [int]
        $Year,

        [switch]
        $asDate = $false
    )
    Begin {
        $alldays = @()
    }
    Process {
        # Validate the Day string passed as parameter by casting it into
        if (-not([System.DayOfWeek]::$Day -in 0..6)) {
            Write-Warning -Message 'Invalid string submitted as Day parameter'
            return
        }

        Switch ($PSCmdlet.ParameterSetName) {
            ByString {
                # Do nothing, variables are already defined and validated
            }
            ByDate {
                $Month = $Date.Month
                $Year = $Date.Year
            }
        }
        # There aren't 32 days in any month so we make sure we iterate through all days in a month
        0..31 | ForEach-Object -Process {
            $evaldate = (Get-Date -Year $Year -Month $Month -Day 1).AddDays($_)
            if ($evaldate.Month -eq $Month) {
                if ($evaldate.DayOfWeek -eq $Day) {
                    $alldays += $evaldate.Day
                }
            }
        }
        # Output
        if ($asDate) {
            Get-Date -Year $Year -Month $Month -Day $alldays[-1]
        }
        else {
            $alldays[-1]
        }
    }
    End {}
}

<#

# Find the last  Wednesday of each month of 2013
1..12 | ForEach-Object -Process {
    Get-LastxOfMonth -Day Wednesday -Month $_ -Year 2013 -asDate
}

# Make a mistake in the spelling of the day name
Get-LastxOfMonth -Day FridayZ -Month 10 -Year 2010

# Get the last Sunday of this month
Get-LastxOfMonth -Day Sunday -Month 10 -Year 2012

# Find the last Friday of the current month but next year
Get-LastxOfMonth -Day Friday -Date (Get-Date).AddYears(1) -asDate

# Submit a Datetime object through the pipeline
(Get-Date).AddMonths(-4).AddYears(1) | Get-LastxOfMonth -Day Friday -asDate

# Use the other parameter set that uses only a value from the pipeline by property name
New-Object -TypeName psobject -Property @{
    Month = 10;
    Year = 2013;
} | Get-LastxOfMonth -Day Friday -asDate

#>